This invention relates generally to the field of devices used to vent gases from containers while retaining other contents within the container, and simultaneously preventing ingress of other undesirable outside substances. More particularly, the invention relates to such devices used with containers containing hazardous materials, such as radioactive waste, where the vent filter must allow passage of hydrogen gas from the containers but prevent radioactive particles from passing through to the atmosphere. Even more particularly, the invention relates to such devices which are multi-stage in construction, with one stage being removable to allow gas samples to be withdrawn through the remaining stage, the removable stage having a filter which absorbs some gases and the remaining stage having a filter which does not absorb any gases.
Safe long-term storage of hazardous waste, and especially of radioactive or mixed chemical/radioactive waste, is a mounting problem. The radioactive materials often produce hydrogen or other gases, meaning that the containers for the waste material must provide for a means to safely vent the gases to prevent explosive build-up within the sealed containers, while at the same time preventing the release of radioactive particulate matter into the atmosphere. In a common situation, the radioactive waste is stored in large metal or polymer drums and a vent filter containing a carbon-composite material is placed into the bung hole or tapped into the wall of the drum. Such filters allow hydrogen gas to pass through while retaining a minimum of 99.93% of the radioactive particulates (to pass Department of Energy regulations), based on a DOP smoke test at 3-5 microns. Other requirements are that the filters allow a minimum flow rate of 35 ml/min based on 1 inch water pressure, with some DOE sites requiring a minimum flow rate of 200 ml/min.
In recent years vent filters substituting a sintered stainless steel media for the carbon composite filter material have been developed. One reason for this change is that it is often necessary or required by the DOE that a sample of the gas in the head space of a drum containing radioactive waste be taken and analyzed prior to shipment of the drum to the final disposal site. Because of the inherent absorption or adsorption properties of the carbon composite filters, the sample cannot be drawn through such a filter without invalidating the sample. The carbon composite must either be removed or destroyed and then replaced, which has the potential of exposing the technicians to the radioactive contents of the drum and creates an additional expense every time a sample is taken. The sintered stainless steel media filters allow passage of hydrogen and other gases, while simultaneously preventing passage of particulate matter. The drum head gas sample can therefore be drawn through the vent filter without invalidating the sample.
The Nuclear Regulatory Commission has approved sintered stainless steel media filters for shipment of radioactive and chemical/radioactive waste to the Waste Isolation Pilot Project in New Mexico. The EPA however is requiring that vent filters for such containers containing chemicals must have the ability to absorb or adsorb and retain certain percentages of volatile organic compounds while still allowing the hydrogen to escape. The sintered stainless steel media filters do not accomplish this, while the carbon composite filters do.
It is an object of this invention to provide a vent filter for a radioactive hazardous waste container which enables a gas sample from the interior of the container to be drawn therethrough without the filter invalidating the sample by absorbing or adsorbing any of the sample, while at the same time preventing passage of radioactive particulates and other matter, and which likewise is able to absorb or adsorb certain volatile organic compounds such as those given off by common solvents. It is a further object to provide such a vent filter which allows single or repeated sampling of the internal gases without destruction of the vent filter. It is a further object to provide such a vent filter which can be, in differing embodiments, secured to an existing threaded bung hole, tapped into a circular aperture or be self-tapping, where the vent filter creates its own aperture. These and other objects not expressed are accomplished by providing a multiple stage vent filter as set forth in detail below, where the vent filter has a first stage adapted to be connected to the waste container and which contains a filter media which allows passage of gases without absorption or adsorption but prevents passage therethrough of particulates, and a removable second stage connected to the first stage, the second stage having an absorbent or adsorbent filter media contained therein which likewise allows passage of gases while preventing passage of particulates, volatile organic compounds and water. To sample the gases within the barrel, the second stage is removed from the first stage, the sample is drawn through the first stage, and the second stage is replaced onto the first stage.